1. Field of the Invention
The invention pertains generally to a fuel management system having an open loop calibration which includes provision for special condition calibrations and is more particularly directed to a special condition calibration for altitude compensation.
2. Prior Art
Electronic fuel schedulers or electronic control units for regulating the air/fuel ratio of an internal combustion engine are conventional in the art. These schedulers provide, from a calculation or electronic computation based upon the operating parameters of the engine, an air/fuel ratio that is considered substantially ideal for the instantaneous conditions sensed.
The "best" air/fuel ratio at which the engine will operate under a given set of operational conditions is normally a tradeoff between the competing factors of driveability, emissions, and fuel economy. It is generally understood that richer air/fuel ratios are better for power and driveability, a substantially stoichiometric air/fuel ratio the most desirable for emissions, and lean air/fuel ratios the calibration that gives the best fuel ecomony. The schedule of desired air/fuel ratios for the electronic control unit can be derived from empirical tests of emissions, driveability, and economy tests and may include areas where the one criterion is more important than the others.
For example, under urban or in city driving, conditions emissions are considered of importance because of the congestion of automobiles present in a small area and the amount of pollutants at these slow speeds while at highway or freeway speeds, ecomony would be the overriding factor of consideration. In addition, for passing or accelerations and to ease starting and warm up situations, power and driveability must be factored into scheduling.
Any number of the various engine parameters may be sensed to calibrate the schedule of air/fuel ratios, but the most advantageous method is to measure mass air flow or mass fuel flow and calculate the other from the schedule.
An air/fuel controller having a calibration based upon the speed of the engine and the density of the air as a measurement of mass air flow has been successfully provided by a U.S. Pat. No. 3,734,068 issued to J. N. Reddy on May 22, l973. The disclosure of Reddy is hereby expressly incorporated by reference herein. Reddy discloses a base calibration pulse width that is a function of the RPM of the engine and manifold absolute pressure. The duration of the pulse width is used to regulate fuel flow to the engine based upon a schedule. This base calibration is an open loop control of the air/fuel ratio as the operating parameters of the engine are sensed by the controller and a control signal which is the fuel pulse duration is developed therefrom.
If the air/fuel ratio schedule from which the control signal is calculated or the engine environment to which it is applied is different from the optimum design system, then the controller will not perform as required. The difference in engine environments are generally either because of manufacturing tolerances that change the response of the engine, or, as occurs with all mechanical devices, the ageing factor which is difficult to schedule.
It is known in the art that to solve many of the problems faced by open loop fuel schedulers a closed loop integral controller may be effectively utilized. The controllers are termed "closed loop" because they sense the result of an actual air/fuel ratio change and develop a control signal based therein rather than calculate an air/fuel ratio change from a desired schedule as does the open loop controller. One of the most advantageous of these controller systems is based upon the bi-level output of an exhaust gas composition sensor which indicates whether a rich or lean air/fuel ratio charge has been combusted by the engine. The controller incrementally leans the air/fuel ratio during a rich indication of the sensor and incrementally enrichens the air/fuel ratio during a lean indication of the sensors, thereby causing the system to oscillate in a limit cycle about a desired air/fuel ratio. Illustrative of this type of controller is a U.S. Pat. No. 3,815,561 issued to Seitz which is commonly assigned with the present application. The disclosure of Seitz is hereby expressly incorporated by reference herein.
One of the special condition calibrations important to such electronic control units is a provision for altitude compensation. As the air density decreases for increases in altitude and the internal combustion engineis operated at an altitude not represented in the schedule, the open loop calibration will be unsatisfactory. One of the primary reasons for this unsatisfactory calibration is the decrease of exhaust gas recirculation at higher altitudes. The amount of exhaust gas recirculation provided in normal open loop systems is a direct function of the difference in intake and exhaust manifold pressures. As the exhaust and intake manifold pressure decrease because of increasing altitude, the EGR valve will recirculate less of the exhaust gas than has been scheduled for. This will cause an automatic lean-out of the calibration as the inducted air/fuel ratio charge will contain more air than the open loop scheduler has anticipated or scheduled. For this reason altitude enrichment is necessitated as a compensating or correcting calibration in terms of the open loop schedule.
It has been established by a previous system that altitude enrichment can be used to compensate for this effect in the power enrichment areas of the manifold pressure portion of the speed density schedule. Since the amount of exhaust gas recirculation that drops out is a direction function of the differential pressure, the effect is more noticeable at these increased manifold pressures near wide-open throttle. Moreover, enrichment is important at these power points for the satisfactory driveability of the internal combustion engine.
The previous system has generally provided a manifold absolute pressure schedule with a power enrichment break point beyond which the air/fuel ratio is decreased more rapidly for increases in manifold pressure than below the break point. Altitude compensation in the power region has been provided by moving the break point downward for increases in altitude, thus starting the power enrichment region of the schedule at lower manifold pressures. This action substantially maintains the relative correspondence between manifold pressure and air/fuel ratio at all altitudes in the power region.
However, with this system it has been found that additional enrichment is required at all manifold absolute pressures as altitude increases and not just in the powerregion of the pressure schedule. The automatic lean-out as altitude increases is apparently due not only to the drop out of EGR in the recirculation loop, but also it is thought that all engines, because of valve overlap and other effects, have a certain percentage of internal EGR which creates this deficiency. It would be, therefore, highly advantageous to provide altitude enrichment for all MAP values of the schedule as altitude increases to calibrate for internal EGR effects and further to retain the altitude enrichment in the power operating curve of the MAP schedule to calibrate for recirculation loop drop out.
When the open loop scheduler is run with a closed loop correction signal from an integral controller and such altitude enrichment is not provided for, a single integrator system will have difficulty in forming a limit cycle as it may reach its authority level and lock up. This will occur when the increase in altitude creates an air/fuel ratio change beyond its authority level. Even with cascaded integral controllers where a secondary integral controller has a higher authority level some difficulty will be observed. Secondary controllers will be constantly approaching some reference level related to altitude. As these controllers are designed to correct for long term scheduling deficiencies and ageing problems, using a portion of their authority level to compensate for altitude significantly restricts their functional capacities. If the altitude compensation is, therefore, completely scheduled for in the open loop portion of the electronic control unit, a closed loop integral controller will be free to perform the function for which it was designed. Further, ramp rates and the authority levels of the integral controllers may be consequently reduced to levels based on parameters other than altitude.